Framing/insulating cathedral ceiling & new partition

Thanks in advance for any help with this.

Basically I need to

1. build a partition in the A-frame to separate space with an insulated barrier in the granier/attic. Dimensions lower base ~40 ft, height, ~40 ft. The framing will need to sit on a rock wall about 3ft thick, it is made of large rocks and limestone mortar. (2nd photo) The stone wall is about 4 ft behind the large wooden A-frame. (1st photo) so there is nothing to directly connect a frame to.

How do you frame something of this size?

Is it possible to use aluminium framing with polystyrene w/1/2" plasterboard backing?

or maybe build a big polystyrene wall from blocks and glue them together? somehow tie the ends to the roof? add plasterboard on after that?

2. insulate and seal the roof without going past the big beams, the beams are ~ 1 ft x ~ 1ft in profile. The roof is clay, "tuille", tiles.

i've read sealed versus ventilated attics but it would be nearly impossible to completely seal these clay tiles. is polystyrene/plasterboard a good way to go here?

as usual, cost sensitive project for some pensioners/retired people on limited income.